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Thread: This time I have questions about sharing resistors for stacked filter caps

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    This time I have questions about sharing resistors for stacked filter caps

    Hello again. Full of questions this morning that I have been trying to figure out for some time.

    Equalizing resistors to share voltage evenly between stacked, rather in-series, filter capacitors.

    At a maximum, the resistor's value should be 50 / capacitor value. So for a pair of 220uF caps in series would have a maximum of 2x 470kΩ resistors, one attached to each capacitor in parallel.

    My first question is to ask if this is correct? Would the calculation be 50/.00011 or 50/.00022? I calculated for 50/.00011 = 454,545Ω

    So if a maximum of 470kΩ resistors can be used, that would suggest that it could also be less than 470kΩ. How low can you go? And more importantly what are the pros vs cons in doing so?? I have read about the speed in which the capacitors are topped off, but is there a benefit to doing this faster or slower?

    Thank you!!!!

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    http://www.valvewizard.co.uk/smoothing.html says 50/C for each capacitor, so in your case:

    Rmax = 50/220e-6 = 227k

    There are many exampled which ignore this though: the 5150ii has to 330” caps in series with 220k balancing resistors, when the 50/C suggests a max of 150k should be used!
    They also use 220k for the 100+100 in the screen supply when they could go as large at 470k. No doubt a case of this is what we've done before so we'll do it again.

    As far what to choose, how long do you want to wait for your caps to bleed off before you can poke inside your amp?

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    Quote Originally Posted by Zozobra View Post
    http://www.valvewizard.co.uk/smoothing.html says 50/C for each capacitor, so in your case:

    Rmax = 50/220e-6 = 227k

    There are many exampled which ignore this though: the 5150ii has to 330” caps in series with 220k balancing resistors, when the 50/C suggests a max of 150k should be used!
    They also use 220k for the 100+100 in the screen supply when they could go as large at 470k. No doubt a case of this is what we've done before so we'll do it again.

    As far what to choose, how long do you want to wait for your caps to bleed off before you can poke inside your amp?
    Right, per capacitor - I have read that actual page a handful of times and when it comes time to calculate those resistances I re-read that section to make sure I am calculating the correct sum.

    And the size has only to do with how fast I want the caps to be bleed? Even using the larger sizes, my amp never sparks when grounding the reservoir cap to the chassis. Except maybe after a very first power on... not sure if that is coincidence or something to do with new caps.

    Thank you!!

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    Nothing wrong with using lower value balancing resistors. It's actually safer with somewhat leaky capacitors (e.g. after longer storing periods). Marshalls used 56k/1W resistors with 100” caps. The only drawback is that lower value resistors waste a little more power acc. to P=VČ/R. We discussed this in more detail in an earlier thread. Don't seem to remember where.

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    The voltage across the series connected capacitors shares according to the capacitors leakage current.
    We therefore size the resistors to conduct 3 to 5 times the expected capacitor leakage current.
    The formula above are derived from this basic requirement.

    Different Capacitor manufacturers specify the leakage current in different ways so we come up with sweeping generalization formula like the ones mentioned above.

    Having actually used real manufacturer datasheet formula to calculate leakage currents and then calculate the right sharing resistors to give 5 times that leakage current I can say that the MAXIMUM resistance value of 50/C is about right but I generally use more like 25/C as the sharing resistor value (e.g.100K for 220uF).

    See Section 4. of this for calcs:
    http://materias.fi.uba.ar/6648/archi...c_appguide.pdf

    Cheers,
    Ian

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    Note that that Application Note I linked above uses the formulae:

    Rshare = 1000/(0.015 x C[uF]) [kOhms]

    That would suggest 303 kOhms MAXIMUM for that 220uF

    This is the same as writing 66.7/C Ohms

    The 50/C says 227 kOhms MAXIMUM for that 220uF so it is conservative and can be used with confidence.


    Cheers,
    Ian

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    Last edited by Gingertube; 10-30-2019 at 06:54 AM.

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    The other aspects worth considering are the max voltage anticipated across each cap in operation, and whether the amp has any existing bleed resistors for B+ (or whatever these caps are for), and the likely upper operating temperature that the caps are sitting in (or may generate themselves if they pass significant ripple current).

    Any balancing resistor will have to withstand the operating voltage (as well as dissipation). And cap leakage will increase significantly with temperature, and probably with age.

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    Quote Originally Posted by Gingertube View Post
    Note that that Application Note I linked above uses the formulae:

    Rshare = 1000/(0.015 x C[uF]) [kOhms]

    That would suggest 303 kOhms MAXIMUM for that 220uF

    This is the same as writing 66.7/C Ohms

    The 50/C says 227 kOhms MAXIMUM for that 220uF so it is conservative and can be used with confidence.


    Cheers,
    Ian
    I was waiting to reply as I was absorbing the document... and I did raise a question flag at that same formula. But all good to know! Thanks

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